Load responsive valve with constant leakage device

ABSTRACT

A pressure compensated load responsive flow control valve for use in a system controlling a plurality of loads. The system is powered by a single, fixed displacement pump. The flow control valve is equipped with a load responsive control, which during simultaneous control of multiple loads automatically maintains the pump discharge pressure at a level higher than the pressure required by the largest load being controlled. To obtain unidirectional flow, load sensing passages of individual valve spools are connected by check valves with the load responsive control, which is equipped with a leakage control, maintaining a relatively constant leakage from load sensing circuit, irrespective of sensing circuit pressure and permitting fast response of pressure compensated load responsive flow control valve without excessive leakage losses.

BACKGROUND OF THE INVENTION

This invention relates generally to pressure compensated load responsiveflow control valves of direction control type, which in control of aload, while using a control load pressure sensing passage, automaticallymaintain pump discharge pressure at a level higher, by a constantpressure differential, than the pressure required by the controlledload, by bypassing excess pump flow to system reservoir. Such a controlvalve disclosed in U.S. Pat. No. 3,488,953 dated Jan. 13, 1970, althougheffective in control of a single positive load at a time, cannotsimultaneously control multiple positive loads.

This disadvantage is overcome by control valve disclosed in my U.S. Pat.No. 3,822,896 and my pending patent application Ser. No. 522,324 filedNov. 8, 1974, entitled "Load Responsive Fluid Control Valves", .Iadd.nowPatent No. 3,998,134 .Iaddend.in which individual check valves, in loadpressure sensing passages, permit phasing pressure signals of only thehighest system load to the differential bypass control of the flowcontrol valve, while isolating pressure signals from other loads. Thosevalves, although effective in control of multiple positive loads, sufferfrom a number of disadvantages. Because of the large cross sectionalarea of the differential bypass valve and its long control stroke, acomparatively large volume of fluid is required to operate it. Thereforesmall diameter and length of load pressure sensing passages, throughwhich the fluid needed for displacement of the differential bypass valvemust pass, limit the response of the valve control and tend to attenuatethe control signal.

The response of the differential bypass valve is also adversely affectedby another factor. Since the displacement of fluid, caused by themovement of the differential bypass valve in one direction tends toclose the check valves in control of load sensing passages, isolatingthe control space filled with fluid, a constant path of leakage must beprovided between the load sensing signal circuit and the systemreservoir. This control leakage is usually obtained by providing anorifice between load sensing circuit and system reservoir. Since flowthrough the orifice is proportional to the square root of pressuredifferential acting across it, and since flow through the orificedetermines response of the differential bypass valve in one direction,an acceptable response of control at low system pressure results in highleakage losses through the control orifice at high system pressure. Thisnot only adversely affects the efficiency of the control valve, butalso, since all of the increased leakage flow must be supplied throughload pressure sensing passages, further attenuates the control signal.

SUMMARY OF THE INVENTION

It is therefore a principal object of this invention to provide acontrol of pressure compensated load responsive flow control valve,which provides fast response of differential bypass valve, whilelimiting maximum control flow from load sensing circuit.

It is another object of this invention to reduce leakage flow from loadsensing circuit to a minimum, while retaining fast response of thedifferential bypass valve.

It is a further object of this invention to provide a pressurecompensated load responsive flow control valve with a leakage controlwhich maintains leakage from load sensing circuit at a relativelyconstant level irrespective of the load sensing circuit pressure andwhich will not largely attenuate control signal transmitted through theload pressure sensing passages of the load sensing circuit.

Briefly the foregoing and other additional objects and advantages ofthis invention are accomplished by providing a differential bypass valveof a load responsive flow control valve with a leakage device whichprovides a relatively constant leakage flow from load sensing circuit,irrespective of load sensing circuit pressure. This feature permits fastuniform response of differential bypass valve throughout the entirepressure range of its operation, without excessive attenuation ofcontrol signal and without excessive leakage at high system pressures.

Additional objects of the invention will become apparent when referringto the preferred embodiments of the invention as shown in theaccompanying drawings and described in the following detaileddescription.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of an embodiment of adifferential bypass valve equipped with constant leakage device used incontrol of flow from schematically shown direction control valve withsystem lines, pump and reservoir shown diagramatically;

FIG. 2 is an enlarged sectional view of leakage spool of FIG. 1; and

FIG. 3 is a sectional view taken along the plane designated by 3--3 ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 shows a section through adifferential bypass valve assembly, generally designated as 10,connected into a circuit with direction control valve assemblies,generally designated as 11 and 12 controlling actuators 13 and 14 whichdrive loads W. Although in FIG. 1, for purposes of demonstration of theprinciple of the invention, differential bypass valve assembly 10 anddirection control valve assemblies 11 and 12 are shown separated, inactual application they would be most likely contained in a single valvehousing or would be bolted together as sections of sectional valveassembly.

As shown, fixed displacement pump 15 has an inlet line 16 which suppliesfluid to pump from a reservoir 17 and the pump is driven through a shaft18 by a prime mover not shown. The pump has an outlet line 19 whichconnects through line 20 to differential bypass valve assembly 10 andthrough lines 21 and 22 with inlet chambers 23 and 24 of directioncontrol valve assemblies 11 and 12 respectively.

Direction control valve 11 has a valve housing 25 which defines outletchambers 26 and 27, which are connected to each other by a duct 28 andare further connected by a line 29 to reservoir 17. Valve housing 25axially guides in a valve bore 30 a valve spool 31 which by lands 32, 33and 34 and stems 35 and 36 defines load chambers 37 and 38, which areconnected through lines 39 and 40 to actuator 13. Load sensing ports 41and 42 are connected through lines 43, 44 and 45 to a check valve 46which in turn is connected by lines 47 and 48 to differential bypassvalve assembly 10.

Similarly, direction control valve assembly 12 has a valve housing 49which defines inlet chamber 24 and also defines outlet chambers 50 and51, which are connected to each other by a duct 52 and further connectedby a line 53 to reservoir 17. Valve housing 49 axially guides in a valvebore 54 a valve spool 55 which by lands 56, 57 and 58 and stems 59 and60 defines load chambers 61 and 62, which are connected through lines 63and 64 to actuator 14. Load pressure sensing ports 65 and 66 areconnected through lines 67, 68 and 69 to a check valve 70, which in turnis connected by line 48 to differential bypass valve assembly 10. Thedifferential bypass valve assembly 10 has a supply chamber 71communicating with pump 15, an exhaust chamber 72 communicating througha line 72a with reservoir 17 and a control chamber 73, those chambersbeing separated by partitions 74 and 75. A bore 76 passing throughpartitions 75 and 74 interconnects supply chamber 71, exhaust chamber 72and control chamber 73 and axially guides a bypass member 77. Bypassmember 77 has an inner bore 78 provided with circumferentially spacedradially extending ports 79 blocked, as shown in position in FIG. 1, bypartition 74 and an extension 80 projecting into control chamber 73.Bypass member 77 is biased by a control spring 81, which maintains it ina position where a stop 82 engages partition 75. Extension 80 isequipped with a variable leakage device, generally designated as 83.Variable leakage device 83 has a leakage spool 84, interposed between astop 85 and a spring guide 86, biased by a spring 87 towards position,as shown in FIG. 1. Stop 85 is provided with openings 85a which allowfree flow of fluid therethrough. Spring 87 is positioned in space 88connected by a drilling 89 with exhaust chamber 72, which in turn isconnected by line 72a with reservoir 17. Preload in spring 87 isadjusted by a threaded member 90. Leakage spool 84, axially guided in abore 91 in extension 80, is equipped with a variable leakage groove 92,shown in dotted lines in FIG. 1 and in section in FIG. 2 and maximumpressure relief grooves 93. Bore 91 of leakage spool 84 terminates atone end in a control surface 94.

All of the basic system components, as shown in FIG. 1, are at rest inunloaded or unactuated position, with fixed displacement pump 15 notworking. With fixed displacement pump 15 not working. With fixeddisplacement pump 15 started up, the pressure in outlet line 19, line 20and supply chamber 71 will start to rise, exerting a force, acting onthe cross sectional area of bypass member 77 within supply chamber 71.As soon as pressure in supply chamber 71 generates a sufficiently highforce on cross sectional area of bypass member 77 to overcome thepreload of control spring 81 bypass member 77 will move from right toleft, trying to displace fluid from control chamber 73 starting to raisethe pressure therein. The resulting rise in pressure in control chamber73 will first close check valves 46 and 70, isolating control chamber 73from direction control valve assemblies 11 and 12. Rising pressure incontrol chamber 73 will induce, in a well known manner, fluid flowthrough variable leakage groove 92 in leakage spool 84 to space 88, fromwhich fluid flow will be conducted through drilling 89 to exhaustchamber 72, which is connected by line 72a with reservoir 17, Resultingflow from control chamber 73 will permit movement of bypass member 77from right to left, the speed of the movement being proportional tofluid flow through variable leakage groove 92 and therefore being afunction of pressure in control chamber 73 and cross sectional area ofbypass member 77.

The movement of bypass member 77 will gradually connect through port 79supply chamber 71 with exhaust chamber 72 and therefore with reservoir17. Under those conditions the fluid supplied by pump 15 to supplychamber 71 will be bypassed to exhaust chamber 72 and condition ofequilibrium will be established under which sufficiently high pressureis maintained in supply chamber 71 to keep bypass member 77 displacedagainst biasing force of control spring 81, to create sufficient flowarea through ports 79 to bypass all of the fluid flow supplied by pump15 to reservoir 17. Therefore under full bypass condition pressure insupply chamber 71 will be equal to the biasing force of control spring81 divided by the cross sectional area of bypass member 77.

The effective area of fluid flow through variable leakage groove 92 willbe established by the relative position of variable leakage groove 92and therefore leakage spool 84 in respect to control surface 94. Inposition, as shown in FIG. 1, with leakage spool 84 maintained againststop 85 by spring 87, the effective area of fluid flow through variableleakage groove 92 will be at its maximum value. Preload in spring 87 isso selected that it will balance a force, generated by a certainspecific pressure in control chamber 73, acting on cross sectional areaof leakage spool 84. Once this pressure level is reached, any furtherincrease in pressure in control chamber 73 will move leakage spool 84from left to right, changing the relative position of variable leakagegroove 92 in respect to control surface 94, changing the effective areaof flow through variable leakage groove 92.

The change in effective flow area through variable leakage groove 92, inrespect to pressure in control chamber 73, will be determined by thecross sectional area of variable leakage groove 92 along the length ofleakage spool and by the preload and rate of spring 87. Since a flowthrough an orifice is proportional to area of orifice and square root ofpressure differential acting across the orifice and since the pressuredown stream of orifice is maintained at relatively constant pressure ofreservoir 17, leakage flow through variable leakage groove 92, inrespect to pressure in control chamber 73, can be regulated in anydesired way to obtain optimum performance of load responsive flowcontrol with minimum control signal attenuation at minimum leakagelevel. The effective flow area of variable leakage groove 92, in respectto movement of leakage spool 84 and rate of spring 87, can be soselected, that a relatively constant leakage flow is maintained fromcontrol chamber 73, irrespective of variation in pressure in controlchamber 73, providing a control with a uniform response in the directionto increase the bypass flow from supply chamber 77 to exhaust chamber72, through the entire pressure range of differential bypass valveassembly 10. As is well known in the art the leakage flow from controlchamber 73 will not only take place through the variable leakage groove92, but also through the working clearance between the outer surface ofthe throttling member 77 and the guiding bore in partition 75. Someadditional leakage will take place around sealing surface of leakagespool 84. Since exhaust chamber 72 is maintained at low pressure of thereservoir 17 and since control chamber is normally maintained at muchhigher pressure, leakage from control chamber 73 through the workingclearance to the exhaust chamber 72 can be high and will vary withpressure in control chamber 73. Therefore the relatively constantleakage flow from control chamber 73, as mentioned above, is maintainedby selection of effective area of variable leakage groove 92 and rate ofspring 87, the actual flow through the variable leakage groove 92reducing with increase in pressure in control chamber 73, to compensatefor other leakages from control chamber 73.

An increase in pressure in control chamber 73 will move bypass member 77from left to right, gradually reducing the area of bypass flow fromsupply chamber 71 to exhaust chamber 72 through ports 79. This reductionin area of flow will increase pressure in supply chamber 71 to a point,at which a condition of equilibrium will be established, at which theforce generated by pressure in control chamber 73, acting on crosssectional area of bypass member 77 together with biasing force ofcontrol spring 81 will be balanced by force generated by pressure insupply chamber 71, acting on cross sectional area of bypass member 77.Therefore differential bypass valve 10 will always regulate bypass flowfrom supply chamber 71 to exhaust chamber 72, to maintain a constantpressure differential between supply chamber 71 and control chamber 73,equal to the preload in control spring 81 divided by the cross sectionalarea of bypass member 77.

Assume that during the equilibrium bypass condition of differentialbypass valve assembly 10, the valve spool 31 of direction control valveassembly 11 is initially displaced from left to right. Displacement ofland 33 connects load chamber 37 with load sensing port 42. Assume alsothat load chamber 37 is subjected to pressure of positive load W,transmitted from actuator 13 through line 39. Load pressure from loadsensing port 42 transmitted through lines 44 and 45, will open checkvalve 46 and pressurize control chamber 73, while maintaining the checkvalve 70 closed. The rising pressure in control chamber 73 will disruptthe equilibrium of forces, acting on bypass member 77, moving it fromleft to right and reducing the area of bypass flow between supplychamber 71 and exhaust chamber 72. In a manner as previously describedthe equilibrium condition of differential bypass valve 10 will bereestablished, the valve maintaining constant pressure differentialbetween supply chamber 71 and control chamber 73 at a new pressure levelin supply chamber 71. Leakage spool 84 will move to its new controllingposition, as dictated by pressure level in control chamber 73,maintaining the required fluid flow from control chamber 73.

Assume that valve spool 31 is further displaced from left to rightconnecting load chamber 37 and load sensing port 42 with inlet chamber23 while at the same time connecting load chamber 38 with outlet chamber27. As previously described inlet chamber 23 is maintained by pump 15 ata pressure, higher by a constant pressure differential, than pressure inload chamber 37. Fluid flow will take place from inlet chamber 23 toload chamber 37, this flow being proportional to the area of openingbetween those two chambers, since a constant pressure differential ismaintained between them. Flow into actuator 13, of fluid supplied by thepump 15, will momentarily lower the pump discharge pressure and disturbthe equilibrium of differential pressure valve assembly 10. As a resultnew bypass position of the bypass member 77 will be established and thedifferential pressure valve assembly 10 will revert to the condition ofequilibrium, at which sufficient quantity of fluid from the pump 15 isbypassed to reservoir 17 by the bypass member 77, to maintain, in amanner as previously described, constant pressure differential betweenload chamber 37 and supply chamber 71. Any sudden reduction in load W,in respect to pressure existing in control chamber 73, willautomatically close check valves 70 and 46. Under those conditionsbypass member 77 will drift from right to left at a rate at which fluidwill flow from control chamber 73 through variable leakage device 83,increasing the bypass flow from supply chamber 71 to exhaust chamber 72and decreasing pressure in supply chamber 71 and consequently controlchamber 73, until check valves 70 and 46 will open connecting pressuresignal from highest system load to control chamber 73. Immediately theequilibrium condition of load responsive control will be reestablished,the control maintaining constant pressure differential between supplychamber 71 and load chamber subjected to highest system load.

Therefore response of load responsive control in the direction ofreduction of system pressure will be determined by the leakagecharacteristics of variable leakage device 83. Any sudden rise in load Wand corresponding increase in pressure in load chamber 37 and thereforecontrol chamber 73 will automatically reposition, in a manner aspreviously described, bypass member 77, to increase the pressure insupply chamber 71 and inlet chamber 23, to establish an equilibriumcondition at which a constant pressure differential is maintainedbetween inlet chamber 23 and load chamber 37. Under those conditions, ina well known manner, flow supplied from the inlet chamber 23 to actuator13 will be proportional to displacement of valve spool 31 from theposition, at which load chamber 37 and inlet chamber 23 becomeconnected.

Displacement of valve spool 31 from right to left will at first connectload sensing port 41 through lines 43, 45, check valve 46 and line 48 tocontrol chamber 73, further movement of valve spool 31 interconnectingload chamber 38 with inlet chamber 23 and also interconnecting loadchamber 37 with outlet chamber 26. The response of the control and thesequence of operations will be the same as those resulting from thedisplacement of the valve spool 31 in the opposite direction and whichhas already been described in detail.

Assume that valve spools 31 and 55 are simultaneously displaced fromleft to right, connecting load sensing ports 42 and 65 with loadchambers 37 and 61. Assume also that pressure of positive load exists inboth load chambers and that load chamber 61 is subjected to higherpressure than load chamber 37. The higher pressure signal from loadchamber 61 will be transmitted through load pressure sensing port 65,lines 68 and 69, check valve 70 and line 48 to control chamber 73. Thehigher load pressure signal from line 48 will also be transmitted byline 47 to check valve 46, in a well known manner, maintaining it closedand therefore isolating load sensing port 42 from control chamber 73.

The response of the system control to high pressure signal in loadcontrol chamber 73 has already been described in detail. However, ifresulting pressure in control chamber 73, due to the system load demandwill exceed a level, at which maximum pressure relief grooves 93, onleakage spool 87, directly cross connect space 88 with control chamber73, large increase in leakage from control chamber 73 will saturate lnes48, 47 and 69 of the load sensing circuit, reducing pressure in controlchamber 73 and creating an unbalance of forces acting on bypass member77, moving it from right to left and reducing the system pressure to thelevel, equivalent to setting of spring 87 and maximum pressure reliefgrooves 93, variable leakage device 83 acting as high pressure pilotrelief valve. To prevent saturation of lines 48, 47 and 69 of the loadsensing circuit and to reduce flow requirement through maximum pressurerelief grooves 93 during operation of variable leakage evice 83 as ahigh pressure pilot relief valve, restriction 47a in line 48 isprovided. When variable leakage device 83 maintains system pressure at aconstant maximum level, the characteristics of the flow control valve,of maintaining constant pressure differential between pump and maximumload pressure are momentarily lost. With drop in load pressure belowthat, equivalent to high flow setting of variable leakage device 83, thevalve control will assume its normal mode of operation. Since duringsimultaneous operation of two loads, the control system will maintain aconstant pressure differential between the pump pressure and thepressure of the highest of the system loads, the flow control feature ofthe lower loads will be lost.

Although preferred embodiments of this invention have been shown anddescribed in detail it is recognized that the invention is not limitedto the precise forms and structure shown and various modifications andrearrangements as will readily occur to those skilled in the art uponfull comprehension of this invention may be resorted to withoutdeparting from the scope of the invention as defined in the claims.

What is claimed is:
 1. A valve assembly comprising a multiplicity of housings, each housing having an inlet chamber, a load chamber subjected to load pressure, an outlet chamber and exhaust means, valve bore means in each housing interconnecting said chambers and axially guiding a valve spool, load sensing port means at the region of each valve bore between said inlet chamber and said load chamber, check valve means operably connected with each of said load sensing port means to permit flow from said load sensing port means to a pressure chamber and to block reverse flow, bypass valve means interconnecting said inlet chambers and said exhaust means to maintain a constant pressure differential between said inlet chamber and said pressure chamber, said bypass valve means including a bypass spool, spring biasing means to bias said bypass spool in one direction to reduce said bypass flow, means responsive to pressure differential betwen pressure in said inlet chambers and pressure in said pressure chamber, said pressure in said pressure chamber being pressure of one of said load chambers subjected to highest load pressure, to bias said bypass spool in opposite direction to increase said bypass flow and pressure responsive variable leakage means continuously interconnecting said pressure chamber and said exhaust means to increase the response of said bypass valve means in a direction to increase said bypass flow, said pressure responsive variable leakage means having variable orifice means varied by pressure in said pressure chamber and subjected to pressure in said pressure chamber said variable leakage means includes spool means having guiding surface means guided in a bore interconnecting said pressure chamber and said exhaust means, varying flow area leakage passage means on said guiding surface means and connecting opposite ends of said spool means, spring biasing means biasing said spool means towards said pressure chamber and positioning said spool means in respect to said bore with change in pressure in said pressure chamber, said varying area leakage passage means cooperating with said interconnecting bore to produce a decrease in leakage flow in response to an increase in pressure in said pressure chamber.
 2. A valve assembly as set forth in claim 1 wherein flow resistance means is interposed between said check valve means operably connected with each of said load sensing port means and said pressure chamber.
 3. .[.A valve assembly as set forth in claim 2 wherein said guiding surface of said spool has.]. .Iadd.A valve assembly comprising a multiplicity of housings, each housing having an inlet chamber, a load chamber subjected to load pressure, an outlet chamber and exhaust means, valve bore means in each housing interconnecting said chambers and axially guiding a valve spool, load sensing port means at the region of each valve bore between said inlet chamber and said load chamber, check valve means operably connected with each of said load sensing port means to permit flow from said load sensing port means to a pressure chamber and to block reverse flow, bypass valve means interconnecting said inlet chambers and said exhaust means to maintain a constant pressure differential between said inlet chamber and said pressure chamber, said bypass valve means including a bypass spool, spring biasing means to bias said bypass spool in one direction to reduce said bypass flow, means responsive to pressure differential between pressure in said inlet chambers and pressure in said pressure chamber, said pressure in said pressure chamber being pressure of one of said load chambers subjected to highest load pressure, to bias said bypass spool in opposite direction to increase said bypass flow and pressure responsive variable leakage means continuously interconnecting said pressure chamber and said exhaust means to increase the response to said bypass valve means in a direction to increase said bypass flow, said pressure responsive variable leakage means having variable orifice means varied by pressure in said pressure chamber and subjected to pressure in said pressure chamber said variable leakage means including spool means having guiding surface means guided in a bore interconnecting said pressure chamber and said exhaust means, varying flow area leakage passage means on said guiding surface means, spring biasing means biasing said spool means towards said pressure chamber and positioning said spool means in respect to said bore with change in pressure in said pressure chamber, said varying area leakage passage means cooperating with said interconnecting bore to produce a decrease in leakage flow in response to an increase in pressure in said pressure chamber, flow resistance means interposed between said check valve means operably connected with each of said load sensing port means and said pressure chamber, .Iadd.and .Iaddend.pressure relief flow area passage means connecting pressure fluid in said pressure chamber with surface of said bore in the region intermediate between said pressure chamber and said exhaust means to interconnect said pressure chamber and said exhaust means at a preselected maximum system pressure in said pressure chamber, said means responsive to pressure differential moving said bypass spool towards position of increased bypass flow to maintain said maximum system pressure at a relatively constant level.
 4. A valve assembly as set forth in claim 3 wherein said variable flow area passage means and said pressure relief area passage means on said guiding surface means of said spool are radially spaced and sealed by said guiding surface means from each other.
 5. A valve assembly comprising at least one housing having an inlet chamber, a load chamber, an outlet chamber and exhaust means, valve bore means in said housing interconnecting said chambers and axially guiding a valve spool, load sensing port means selectively communicable with said load chamber by said valve spool, bypass valve means interconnecting said inlet chamber and said exhaust means operable to bypass fluid between said inlet chamber and said exhaust means to maintain a constant pressure differential between said inlet chamber and said load sensing port means, said bypass valve means including a bypass spool, spring biasing means to bias said bypass spool in one direction to reduce said bypass flow, means responsive to pressure differential between said inlet chamber and said load sensing port means to bias said bypass pool in opposite direction to increase said bypass flow and a pressure responsive variable leakage means continuously interconnecting said load sensing .[.pot.]. .Iadd.port .Iaddend.means and said exhaust means to increase the response of said bypass valve means in direction to increase said bypass flow, said pressure responsive variable leakage means having variable orifice means varied by pressure in said pressure chamber and subjected to pressure in said pressure chamber, said variable leakage means includes spool means having guiding surface means guided in a bore interconnecting said pressure chamber and said exhaust means, varying flow area leakage passage means on said guiding surface means and connecting opposite ends of said spool means, spring biasing means biasing said spool means towards said pressure chamber and positioning said spool means in respect to said bore with change in pressure in said pressure chamber, said varying area leakage passage means cooperating with said interconnecting bore to produce a decrease in leakage flow in response to an increase in pressure in said pressure chamber. .Iadd.
 6. A valve assembly comprising at least one housing having an inlet chamber, a load chamber, an outlet chamber and exhaust means, valve bore means in said housing interconnecting said chambers and axially guiding a valve spool, load sensing port means selectively communicable with said load chamber by said valve spool, bypass valve means interconnecting said inlet chamber and said exhaust means operable to bypass fluid between said inlet chamber and said exhaust means to maintain a constant pressure differential between said inlet chamber and said load sensing port means, said bypass valve means including a bypass spool, spring biasing means to bias said bypass spool in one direction to reduce said bypass flow, means responsive to pressure differential between said inlet chamber and said load sensing port means to bias said bypass spool in opposite direction to increase said bypass flow, first constant area leakage means continuously interconnecting said load sensing port means and said exhaust means, and a second pressure responsive variable leakage means continuously interconnecting said load sensing port means and said exhaust means operable to increase the response of said bypass valve means in direction to increase said bypass flow, said second pressure responsive variable leakage means having variable orifice means varied by pressure in said pressure chamber and subjected to pressure in said pressure chamber, said variable leakage means includes spool means having guiding surface means guided in a bore interconnecting said pressure chamber and said exhaust means, varying flow area leakage passage means on said guiding surface means, spring biasing means biasing said spool means towards said pressure chamber and positioning said spool means in respect to said bore with change in pressure chamber, said varying area leakage passage means cooperating with said interconnecting bore to decrease in a preselected way leakage flow in response to an increase in pressure in said pressure chamber whereby the sum of the leakage through said first and said second leakage means remains relatively constant irrespective of the change in pressure in said load sensing port means. .Iaddend..Iadd.
 7. A valve assembly comprising a multiplicity of housings, each housing having an inlet chamber connected to a pump, a load chamber subjected to load pressure, an outlet chamber and exhaust means, valve spool means for selectively interconnecting said chambers, load sensing port means selectively communicable with said load chambers by said valve spool means, check valve means operably connected with each of said load sensing port means to permit flow from said load sensing port means to a pressure chamber and to block reverse flow, pump output flow control means responsive to pressure in said pressure chamber and operable to maintain a relatively constant pressure differential between discharge pressure of said pump and pressure in said pressure chamber, first constant area leakage means continuously interconnecting said pressure chamber and said exhaust means, and second pressure responsive variable leakage means continuously interconnecting said pressure chamber and said exhaust means, said second pressure responsive variable leakage means having variable orifice means varied by pressure in said pressure chamber and subjected to pressure in said pressure chamber, said variable leakage means including spool means having guiding surface means guided in a bore interconnecting said pressure chamber and said exhaust means, varying flow area leakage passage means on said guiding surface means, spring biasing means biasing said spool means towards said pressure chamber and positioning said spool means in respect to said bore with change in pressure in said pressure chamber, said varying area leakage passage means cooperating with said interconnecting bore to decrease in a preselected way leakage flow in response to an increase in pressure in said pressure chamber whereby the sum of the leakage through said first and said second leakage means remains relatively constant irrespective of the change in pressure in said pressure chamber.
 8. A valve assembly as set forth in claim 7 wherein flow resistance means is interposed between said check valve means operably connected with each of said load sensing port means and said pressure chamber. .Iaddend. .Iadd.
 9. A control assembly comprising a housing having a first chamber subjected to a first variable fluid pressure level and supplied through a fluid conducting means having fluid flow resistance means, and a second chamber subjected to a second fluid pressure level, said first variable fluid pressure level being higher than said second fluid pressure level, first constant area leakage means continuously interconnecting said first and said second chambers, second variable leakage means continuously interconnecting said first and said second chamber having means responsive to pressure differential between said chambers, said second variable leakage means having variable orifice means varied by pressure differential between said chambers, said variable leakage means including spool means having guiding surface means guided in a bore interconnecting said first and second chambers, variable flow area leakage passage means on said guiding surface means, spring biasing means biasing said spool means towards said first chamber and positioning said spool means in respect to said bore with change in pressure differential between said first and second chambers, said varying flow area leakage passage means cooperating with said interconnecting bore to decrease in a preselected way leakage flow between said first and second chambers in response to increase in pressure differential between said chambers whereby the sum of the leakage through said first and said second leakage means remains relatively constant irrespective of the change in pressure differential between said first and said second chambers. .Iaddend. 